Bit rates of 10 Gbit/s per channel are customary today for optical wavelength multiplex systems. Forty-Gbit/s systems have also been developed and implemented as models, but these systems are very demanding in terms of expenditure. Particular problems give rise therein to distortions in the transmitted signals such as polarization mode dispersion (PMD) and chromatic dispersion. Polarization controllers and optical compensation devices are used to compensate said distortions.
Numerous modulation and encoding methods are known from the communications field and numerous possibilities have been investigated to find suitable methods leading to a significant improvement in the transmission characteristics of optical systems.
In “telcom report” 1/88, pages 22 to 25, a radio relay system is described that transmits orthogonally polarized signals by means of suitable antennas. Imperfect technical equipment such as poorly oriented antennas as well as reflections give rise to cross-polarization faults where one signal overlaps another actually orthogonal signal in a disruptive manner. The polarized signals are split at the receiving end by separate antennas. Adaptive time-domain equalizers are provided for eliminating cross-polarization faults. The basic concept of depolarization compensation is to obtain a compensation signal and add it to the respective main signal. No frequency errors must occur and any inter-signal time displacements must be compensated. The change in polarization occurring in a radio relay system is, however, slight compared to optical transmission over fibers. Any polarization can occur here.
An arrangement and a method for transmitting polarization multiplex signals are described in the publication of an unexamined application reference DE 101 56 244 A1. A differential phase modulation is applied at the transmitting end between the orthogonally polarized transmission signals. Said modulation serves to control a common polarization transformer, or to control separate polarization transformers for each transmission channel, by evaluating interferences. The purpose thereof is to maximize the control rate with a minimal expenditure requirement. The polarization transformer can be embodied for compensating (lacuna). The transmission link is for this purpose simulated with the aid of a double-refracting crystal and employing numerous control voltages as being “inverse”, so that the pulse mode dispersion is compensated. The measures taken at the transmitting end are frequently undesirable for reasons of transparency; moreover, both fast and reliable PMD controlling in the optical range remains demanding in terms of expenditure.